Optical disc apparatus with rapid and stable accessing capability

ABSTRACT

An optical disc apparatus comprising an optical servo loop and a velocity servo loop to control the velocity of a pickup to zero, wherein the two servo loops are selectively switched in accordance with the set mode of disc playing or the state of such playing. When a fast access is executed, the pickup is controlled by the velocity servo loop to avert undesired vibration in the travel of the pickup and also to protect the optical servo from any shock that may otherwise be caused by the travel and halt of the pickup. And in playing of any disc having a great amount of dropouts, the velocity servo loop is closed during the existence of such dropouts to keep the optical servo loop from the disturbance signal generated due to the dropout, thereby ensuring stable reproduction of signals from the disc.

This is a continuation application of Ser. No. 07/209,642, filed June21, 1988, pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to an optical disc apparatus and, moreparticularly, to an apparatus for reproducing signals or data recordedon a compact disc, video disc or the like.

2. Description of the Prior Art:

In an optical disc player, it is generally customary that a light beamis positioned by cooperation of a rough actuator which comprises alinear motor and so forth for driving a pickup-base, and a fine actuatorfor driving a focus lens mounted on the pickup-base.

For example, when an access is executed in the ordinary optical discplayer, first the pickup-base is positioned approximately at a desiredtarget point by the rough actuator, and then the fine actuator mountedon the pickup-base is driven under fine control to position the focuslens exactly on the target track.

However, in case the rough actuator is driven at a high speed for a fastaccess, the focus lens held by the fine actuator is deflected due to theacceleration caused at the start or halt of the rough actuator or by thevibration induced during the travel thereof, so that the fine actuatoris kept in vibration for a while even after the halt of the pickup-basein the vicinity of the target, thereby raising a problem that the fineactuator cannot be brought into servo control with stability andrapidity.

In an attempt to solve the above problem, there is proposed an improvedsystem wherein a position detector is disposed in the vicinity of thefine actuator, and a position servo loop applying the output signal ofthe position detector as an error signal is formed during the travel ofthe pickup-base to lock the fine actuator at its mechanically neutralpoint, thereby preventing vibration of the fine actuator.

However, there still remain some problems unsolved with regard tocomplication in the structure of the position detector and difficultiesin constituting a stable position servo circuit due to the d-c drift inthe component elements of the position servo circuit, and also thesecular change caused therein such as temperature fluctuation inparticular.

For the purpose of eliminating the above-described drawbacks existing informing a stable position servo circuit, an improvement has beenaccomplished as disclosed in Japanese Patent Laid-open No. 61(1986)-214230, wherein a position servo loop is so formed that theposition of the fine actuator immediately before a fast displacement ofthe pickup-base is stored in a hold circuit and the fine actuator isretained at such position during the travel of the pickup-base.

According to the above system, a position detector is disposed in thevicinity of the fine actuator to produce an output serving as a positionerror signal, and a position servo loop is formed in such a manner as touse, as a reference servo signal, the position error signal obtainedfrom the position detector and stored in the hold circuit immediatelybefore the fast displacement of the pickup-base, so that despite anyvariation in the characteristic of the position detector, the servocontrol is effected while such variation is absorbed to consequentlysettle the problems including the aforementioned d-c drift, secularchange and temperature fluctuation.

In the above system, however, it becomes necessary to employ a holdcircuit for storing the position data therein, hence failing in solutionof the problem with regard to the complicated structure of theapparatus.

Furthermore, according to the above system, a reference position signalof the position servo loop is derived from the position error signalobtained by holding the position error output, which is produced fromthe position detector disposed in the vicinity of the fine actuator, atthe value immediately before start of the travel of the pickup-base.Therefore, although it may be possible to solve the problem of vibrationof the pickup-base during its travel, another disadvantage is stillexistent that, if the pickup-base begins its travel with the fineactuator inclined due to some eccentricity of the disc or the like, thefine actuator is driven continuously while being retained in suchinclined posture. And when the travel of the pickup-base is brought to ahalt, the fine actuator is still kept in the inclined posture and thetracking servo mode is selected in such state, so that the operation islimited by the dynamic range of the servo circuit and the mechanicaldynamic range of the fine actuator to eventually bring about instabilitywith respect to the tracking servo pull-in action.

Moreover, upon occurrence of any dropout during reproduction of signalsfrom a disc (or playing of a disc) in the conventional optical discplayer, a great disturbance signal is generated in each of the errorsignals in the focus servo loop and the tracking servo loop formed tocontrol the fine actuator which supports the focus lens therein, hencecausing a trouble that the focus lens is positionally deviated.

In an attempt to solve this problem, an improved invention has beencontrived as described in Japanese Patent Laid-open No. 59 (1984)-38980.According to the technique disclosed, in reproduction of signals fromany disc having many dropouts, the gain of each servo loop is lowered bymeans of a manual switch to relatively reduce the amplitude of thedisturbance signal generated during the occurrence of such dropouts,thereby minimizing the deviation of the focus lens.

But an important problem is still left unsolved in the system mentionedabove that, when setting a stable servo gain for each loop, it becomesnecessary to previously find a disc having many dropouts (as the amountof dropouts is unknown until signals recorded on the disc is actuallyreproduced in the player) and, even with proper setting of an optimalgain by reduction of its value, there occurs an adverse side effect thatthe operation is influenced readily by external mechanical vibration.

There is further known another system with regard to a countermeasurefor such dropout, wherein, during generation of the dropout detected bysome other means, the servo loop is opened so that the disturbancesignal is not transmitted to the fine actuator, thereby preventing apositional deviation of the focus lens.

The system mentioned above is capable of relatively diminishing theadverse side effect that the operation is influenced by the externalvibration, since the servo loop is opened merely for the moment of thedropout.

However, because of the structure where the focus lens is supported by amechanical damper, the restoring force derived from the elasticity ofthe damper itself is exerted to move the focus lens which is placed in anoncontrolled state during generation of the dropout. Consequently, atthe termination of the dropout, the focus lens thus moved is on anothertrack spaced apart from the desired former track prior to the dropout.

Furthermore, in some of the laser disc players manufactured in arelatively early stage of the technical development, the focus lens isnot furnished with a mechanical damper in regard to the focuscontrolling direction and is supported merely by electromagnetic means(focus servo) alone, so that the position of the focus lens is varieddue to the gravity during the dropout period and a great servo errorsignal corresponding to such positional deviation is generated at thetermination of the dropout to eventually bring about instability of theoperation.

SUMMARY OF THE INVENTION

In the optical disc apparatus of the present invention, an optical servoloop is formed for positioning a light spot on a disc with a fineactuator which holds a focus lens therein and is mounted on a roughactuator. Furthermore, means for detecting the motion of the fineactuator is provided and, in addition to the optical servo loop, avelocity servo loop is formed for extracting a velocity signal from theoutput of such detecting means and feeding the velocity signal back tothe fine actuator, wherein the two servo loops are selectively switchedin accordance with the set mode of disc reproduction or the state ofsuch reproduction.

It is a first object of the present invention to suppress vibration ofthe fine actuator, where the focus lens is held, during the travel of apickup-base driven by the rough actuator, so as to execute an accesswith rapidity and stability.

A second object of the invention resides in achieving a stable servopull-in action within a short time when leading the operation into theoptical servo loop again after the fast travel of the pickup-base.

A third object of the invention is to perform stable reproduction ofsignals or data from any disc having an extremely great amount ofdropouts.

For the purpose of attaining the first object, the optical servo loop(tracking servo loop and/or focus servo loop) is opened during the fasttravel of the pickup-base driven by the rough actuator, andsimultaneously the velocity servo loop is closed with a referencevelocity set to zero, thereby suppressing the vibration of the focuslens held in the fine actuator.

In order to attain the second object, the fine actuator is placed in anoncontrolled state for a predetermined time after completion of thefast travel of the pickup-base driven by the rough actuator, and thenthe optical servo loop is closed again; or the velocity servo loop isclosed for a fixed time after the fine actuator is placed in anoncontrolled state for a predetermined time, and subsequently theformer optical servo loop is closed again.

To attain the third object, detection means is provided for detectingany dropout on the disc, and the optical servo loop is opened during theexistence of such dropout while the velocity servo loop is closed withthe reference velocity set to zero, thereby locking the motion of thefine actuator to prevent the focus lens from being driven by anyservo-disturbing error signal generated during the dropout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a first embodiment of the presentinvention;

FIG. 2 (a, b, c) is a timing chart showing the states of individualservo loops in the first embodiment of the invention;

FIG. 3 illustrates the behavior of a focus lens in a second embodimentof the invention;

FIG. 4 (a, b, c, d) is a timing chart showing the states of individualservo loops in the second embodiment of the invention;

FIG. 5 (a, b, c, d, e, f) graphically shows the operation of the focuslens in the second embodiment of the invention;

FIG. 6 is a block diagram of a third embodiment of the invention; and

FIG. 7 (A), FIG. 7 (B) and FIG. 7 (C) illustrate the operationsperformed upon occurrence of a dropout in the third embodiment of theinvention, in which (A) represents an example without any countermeasurefor the dropout, (B) represents an example with a conventionalcountermeasure, and (C) represents another example with thecountermeasure according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram showing the basic constitution of an opticaldisc apparatus embodying the present invention. In this diagram areincluded a disc 1, and a pickup 2 which comprises a focus lens 2a, adamper 2b for supporting the focus lens 2a, a drive coil 2c for drivingthe focus lens 2a in a tracking direction, and a tracking error detector2d for detecting a tracking error.

The damper 2b and the drive coil 2c constitute a fine actuator. Actuallythe fine actuator includes another set of a damper and a drive coil fordriving the focus lens in a focusing direction, which are not shown inthe figure to avoid complexity.

There are also shown a pickup-base 3 for mounting the pickup 2 thereon,a linear motor 4 for driving the pickup-base 3, and a position detector5 fixedly mounted on the pickup-base 3 for detecting, in the trackingdirection, the position of the focus lens 2a. The position output signalobtained from the position detector 5 is converted to a velocity signalVv by a differential circuit 6 consisting of a capacitor 6a and aresistor 6b.

The velocity signal Vv is fed to a subtractor 8, which compares theinput signal V_(v) with a reference velocity signal V_(s) thereby togenerate a velocity error signal Ve. The velocity error signal Ve thusobtained is fed to a power amplifier 10 via a phase compensation circuit9 for advancing the phase in a higher frequency range and a selectorswitch 12 controlled by an access order signal As, and the output of theamplifier 10 is then fed to the drive coil 2c.

A velocity servo loop is constituted by the above-described positiondetector 5, differential circuit 6, subtractor 8, phase compensationcircuit 9, selector switch 12, power amplifier 10 and drive coil 2c.

In such velocity servo loop, the velocity of the focus lens 2a can becontrolled to be zero, or locked, by setting the reference velocitysignal Vs to zero.

Reference numeral 11 denotes a phase compensation circuit forstabilizing the servo control by advancing the phase in a higherfrequency range of the output signal of the tracking error detector 2d.A tracking servo loop is constituted by the above-described trackingerror detector 2d, phase compensation circuit 11, selector switch 12,power amplifier 10 and drive coil 2c.

Such tracking servo loop enables a light beam, which is outputted fromthe pickup 2, to exactly follow a desired track on the disc 1.

Denoted by reference numeral 14 is a pickup-base control circuit which,in a normal reproduction mode, amplifies the output signal TE of thetracking error detector 2d and feeds the amplified signal to the linearmotor 4 or, when moving the pickup-base 3 fast, feeds a fullacceleration signal to the linear motor 4. The operation of the controlcircuit 14 is selectively switched by the access order signal At.

A traverse control circuit is constituted by the above-describedtracking error detector 2d, pickup-base control circuit 14 and linearmotor 4 for controlling, in the normal reproduction mode, the motion ofthe pickup-base 3 in its radial direction.

Although there is not shown in FIG. 1 a block of a focus servo toperform focus control between the disc 1 and the focus lens 2a, it isassumed here that the focus servo control is continuously effected withstability.

FIG. 2 is a timing chart showing the states of the individual servoloops and the relationship therebetween when an access is executedduring reproduction of signals or data from a disc in the presentinvention.

In FIG. 2, (a) and (b) represent the state of the tracking servo loopand that of the velocity servo loop, respectively; and (c) representsthe position of connection of the selector switch 12. Furthermore, A andB indicate an access start point and an access end point, respectively.

In the timing chart of FIG. 2, the tracking servo loop is switched offor opened during the access time (τ0) between the points A and Bcorresponding to the fast travel of the pickup-base 3, and the velocityservo loop is switched on or closed with the reference velocity signalVs set to zero. Accordingly the focus lens 2a is locked to thepickup-base 3 until the point B where the fast travel of the pickup-base3 is terminated, and the velocity servo loop is switched over to thetracking servo loop in a state where the focus lens 2a is keptrelatively still, so that proper servo pull-in can be executed withstability in a short period of time.

Since the focus lens 2a is locked to the pickup-base 3 during a roughaccess, if any vibration of the focus lens 2a is caused by a greatacceleration derived from start or halt of the rough access or by thevibration generated during the travel of the pickup-base 3, the velocityis controlled to zero by the velocity servo loop to consequentlysuppress such vibration of the focus lens 2a, whereby the tracking servocontrol can be effected with stability immediately after completion ofthe rough access.

FIGS. 3, 4 and 5 relates to a second embodiment of the optical discapparatus according to the present invention, explaining the operationto control the position of the focus lens 2a to its optimal state forresuming the tracking servo control with rapidity and stabilityimmediately after the fast travel of the pickup-base 3.

The basic block constitution of the second embodiment is the same asthat of the first embodiment shown in FIG. 1.

FIG. 3 illustrates the states of the focus lens 2a immediately beforeand after a fast access. FIG. 4 (a), (b), (c) and (d) are timing chartsrespectively showing the motion of the focus lens 2a, the state of thetracking servo loop, the state of the velocity servo loop, and the stateof connection of the selector switch 12. FIG. 5 graphically shows thebehavior of the focus lens 2a when it is suddenly shifted from aninclined state to a free state.

Hereinafter the operation of the second embodiment will be described indetail with reference to FIGS. 3 through 5.

In FIG. 3, A denotes an access start point. Suppose now that the focuslens 2a starts its travel from the point A together with the pickup-base3 while being locked in an inclined posture at an angle θ due to theeccentricity of the disc.

Upon arrival of the pickup-base 3 at a point B after its fast travel,the servo loop is opened for a predetermined time, and then the focuslens 2a is urged to automatically return to its mechanically neutralpoint by the elasticity of the damper 2b, so that proper tracking can beachieved by closing the tracking servo loop in accordance with a shiftof the focus lens 2a to the point of its upright posture, i.e., from aposition 2 to another position 3.

In the timing chart of FIG. 4, the selector switch 12 is connected tothe side a during the travel of the pickup-base 3 (corresponding to timeτ0) to form the velocity servo loop with the reference velocity Vs setto zero, so that the inclination of the focus lens 2a is retained asrepresented by 1 and 2 in the chart.

During the next time τ1, the switch 12 is connected to the side c toopen the entire servo loops, whereby the focus lens 2a is rendered freeand then is returned automatically to its mechanically neutral point (3in FIG. 4) by the elasticity of the damper 2b.

Subsequently the switch 12 is connected to the side a again during thenext time τ₂, so that the velocity of the focus lens 2a caused by themotion of the damper 2b is controlled to zero at the position 3 of theupright posture, and therefore the focus lens 2a is brought to a halt tobe ready for re-pulling in the tracking servo.

Due to the series of operations mentioned above, the focus lens 2a isretained in its upright posture and the velocity thereof can becontrolled to zero even immediately after the fast drive of thepickup-base 3, whereby the tracking servo control can be resumed withrapidity and stability.

FIG. 5 is a graphic representation for explaining the length of the timeτ1 during which the entire servo loops are opened (with the selectorswitch 12 connected to the side c) in the operation described above.

FIG. 5 indicates that when the focus lens 2a is suddenly rendered freefrom the inclined state a, its inclination becomes zero at each ofpoints b, d, f and so forth.

Although the focus lens 2a is thus retained upright at any of the pointsb, d, f and so forth, it is desired that normally the focus lens 2a beset at the first upright point b since the earlier the timing to pull inthe tracking servo, the better the result attained.

Relative to the resonance frequency f0 (Hz) of the damper 2b for holdingthe focus lens 2a therein, the time of its one period is approximately1/f0 (which is slightly varied by the damping factor of the resonancesystem of the damper 2b including the focus lens 2a). Therefore, in thisexample, the time τ1 for opening the entire servo loops comes to beabout 1/4·f0 (sec).

It follows that, after the travel of the pickup-base 3, the focus lens2a is rendered free for the time τ1 which is substantially equal to 1/4of the resonance-frequency period of the pickup 2 including the focuslens 2a, so that it becomes possible to attain high stability whenpulling in the tracking servo.

The length of the time τ2 is determined by the gain of the velocityservo loop, and it can be reduced to a sufficiently small value which isnormally less than several milliseconds.

In the second embodiment mentioned, after the focus lens 2a is placed inits noncontrolled free state for the time τ1, the velocity servo loop isclosed merely during the time τ2 for reducing the velocity of the focuslens 2a to zero, and then the tracking servo loop is closed. However, incase the gain of the tracking servo loop is sufficiently high, a stablepull-in action is possible even when the focus lens 2a has a velocity,so that the time τ2 for closing the velocity servo loop may be omittedto execute direct pull-in of the tracking servo.

FIG. 6 is a block diagram of a third embodiment of the optical discapparatus according to the present invention, and it is so constitutedas to solve the known problem that, upon occurrence of any dropoutduring reproduction mode, the focus lens is deviated by a greatdisturbance signal generated in the servo loop (in the focusing ortracking direction).

The constitution of FIG. 6 is accomplished by providing, in addition tothe aforementioned first embodiment of FIG. 1, a dropout detectioncircuit 13 for detecting dropout data from the tracking error signal TE,and a switch control circuit 15 for combining the access order signal Aswith the output of the dropout detection circuit 13 and controlling theselector switch 12 by such combined signal.

In the third embodiment, the access order signal serves to directlycontrol the switch 12 via the switch control circuit 15, so that theoperation performed in this case is exactly the same as that in theforegoing second embodiment.

FIG. 7 illustrates the behavior of the focus lens 2a upon occurrence ofany dropout in the reproduction mode, so as to explain the effect of thethird embodiment of the present invention.

FIG. 7 (A) represents an exemplary case where no particularcountermeasure is taken, i.e. none of dropouts is detected and the servoloops are not switched either.

FIG. 7 (B) represents another case where a signal is generated bydetecting the occurrence of a drop-out, and the tracking servo loop isopened during the time τ of such dropout in response to the detectionsignal, thereby placing the focus lens 2a in a noncontrolled free stateto minimize the harmful influence derived from disorder of the trackingerror signal. This operation is based on one of the conventionaltechniques known heretofore.

And FIG. 7 (C), which corresponds to the third embodiment of the presentinvention, represents a further case where the tracking servo loop isopened during the dropout time τ and simultaneously the velocity servoloop is closed with the reference velocity Vs set to zero.

In FIG. 7 (A), (B) and (C): (a) shows the waveform of an RF signaldetected from the disc by an RF signal detector omitted in the blockdiagram of FIG. 6; (b) shows the waveform of a tracking error signal TE;(c) shows the waveform of a dropout detection signal D0 outputted fromthe dropout detection circuit 13; (d) shows the state of the trackingservo loop; (e) shows the state of the velocity servo loop; and (f)shows the behavior of the focus lens 2a.

In the first case of FIG. 7 (A) where no counter-measure is taken, acomponent Tg different from the original tracking signal is generated inthe tracking error signal TE during the dropout time (signifying in thisembodiment the time period in which the tracking error signal TE isdisordered to a great extent). Consequently, there arises a trouble thatthe focus lens 2a is deflected widely by such disturbance component Tgand, at the instant of termination of the dropout, the focus lens 2a isdeviated far from the original track and is not returnable to the formerstate.

In the conventional system of FIG. 7 (B) where the tracking servo loopis opened during the dropout time, if any disturbance component Tg notincluded in the original tracking signal is generated, the selectorswitch 12 is controlled by the dropout detection signal D0 of thedropout detection circuit 13 via the switch control circuit 15, so thatthe switch 12 is connected to the side c to open the entire servo loops,whereby the disturbance component Tg is not transmitted to the coil 2cfor driving the focus lens 2a, which is consequently not deviated far asin the foregoing example of FIG. 7 (A).

However, the damper 2b holding the focus lens 2a therein has a specificelasticity and therefore exerts a mechanical restoring force on thefocus lens 2a, which is thereby urged to automatically return to itsmechanical neutral point. In most cases, accordingly, there arises aproblem that a positional discrepancy is induced between the position ofthe focus lens 2a at the dropout start instant and that at the dropoutend instant.

That is, at the instant of termination of the dropout, tracking servo iseffected on another track spaced apart slightly from the desired track,and consequently there also occurs a deviation of the light beam.

According to the present invention of FIG. 7 (C), the selector switch 12is controlled by the dropout detection signal D0 during the dropout timeτ in such a manner as to be connected to the side a to form the velocityservo loop with the reference velocity Vs set to zero, so that thevelocity of the focus lens 2a in the tracking direction is maintained atzero. In other words, the focus lens 2a is positionally locked andretained in this case, and therefore its position at the termination ofthe dropout remains unchanged in comparison with that at the beginningof the dropout. As a result, despite sudden switchover to the trackingservo loop, the tracking servo control can be resumed on the same trackwith stability.

In any of the exemplary embodiments of the present invention explainedhereinabove, the description has been given with respect to theoperation performed in the tracking direction. It is to be understood,however, that the present invention is not limited to the above examplesalone, and similar effects are also achievable with regard to thecontrol action in the focusing direction as well. Since the operation inthe focusing direction is fundamentally the same as that in the trackingdirection, a repeated explanation is omitted here.

What is claimed is:
 1. An optical disc apparatus comprising:pickup meansfor forming a light spot on an optical disc and detecting a light fromsaid optical disc, said pickup means including (a) a focus lens, and (b)a fine actuator for movably supporting thereon said focus lens andmoving said focus lens; light spot positioning means for positioningsaid light spot on said optical disc, said light spot positioning meansconstituting an optical servo loop including (a) position errordetection means for detecting a position error of said light spot andproducing a position error signal indicative of the detected positionerror of said light spot, (b) first phase compensation means forcompensating a phase of said position error signal to obtain aphase-compensated position error signal, and (c) means for feeding saidphase-compensated position error signal to said fine actuator;traversing means for moving said pickup means in a radial direction ofsaid optical disc, said traversing means being responsive to anexternally given control signal for moving said pickup means at a highspeed; velocity control means for controlling a velocity of said focuslens, said velocity control means constituting a velocity servo loopincluding (a) velocity detection means for detecting a velocity of saidfocus lens and producing a velocity signal indicative of the velocity ofsaid focus lens, (b) velocity error signal producing means for comparingthe velocity signal with a predetermined reference velocity to producean error therebetween as a velocity error signal, (c) second phasecompensation means for compensating a phase of the velocity error signalto obtain a phase-compensated velocity error signal, and (d) means forfeeding said phase-compensated velocity error signal to said fineactuator; and switching means responsive to an externally given controlsignal for selectively closing one of said optical servo loop and saidvelocity servo loop.
 2. An optical disc apparatus according to claim 1,wherein said reference velocity is set to zero in said velocity servoloop.
 3. An optical disc apparatus according to claim 1, wherein saidvelocity servo loop includes at least one of a focusing-directionvelocity servo loop and a tracking-direction velocity servo loop.
 4. Anoptical disc apparatus according to claim 1, wherein said velocitydetection means comprises a position detector for detecting a positionof said focus lens, and a differentiation circuit for blocking through acapacitor one of the direct-current component and ultralow-frequencycomponent in an output of said position detector.
 5. An optical discapparatus comprising:pickup means for forming a light spot on an opticaldisc and detecting a light from said optical disc, said pickup meansincluding (a) a focus lens, and (b) a fine actuator for movablysupporting thereon said focus lens and moving said focus lens; lightspot positioning means for positioning said light spot on said opticaldisc, said light spot positioning means constituting an optical servoloop including (a) position error detection means for detecting aposition error of said light spot and producing a position error signalindicative of the detected position error of said light spot, (b) firstphase compensation means for compensating a phase of said position errorsignal to obtain a phase-compensated position error signal, and (c)means for feeding said phase-compensated position error signal to saidfine actuator; traversing means for moving said pickup means in a radialdirection of said optical disc, said traversing means being responsiveto an externally given access order signal applied thereto in an accessoperation mode for moving said pickup means at a high speed; velocitycontrol means for controlling a velocity of said focus lens, saidvelocity control means constituting a velocity servo loop including (a)velocity detection means for detecting a velocity of said focus lens andproducing a velocity signal indicative of the velocity of said focuslens, (b) velocity error signal producing means for comparing thevelocity signal with a predetermined reference velocity to produce anerror therebetween as a velocity error signal, (c) second phasecompensation means for compensating a phase of the velocity error signalto obtain a phase-compensated velocity error signal, and (d) means forfeeding said phase-compensated velocity error signal to said fineactuator; and switching means for selectively closing one of saidoptical servo loop and said velocity servo loop, said switching meansbeing responsive to an externally given access order signal appliedthereto in the access operation mode for opening said optical servo loopand closing said velocity servo loop.
 6. An optical disc apparatusaccording to claim 5, wherein said reference velocity is set to zero insaid velocity servo loop.
 7. An optical disc apparatus according toclaim 5, wherein said velocity servo loop includes at least one of afocusing-direction velocity servo loop and a tracking-direction velocityservo loop.
 8. An optical disc apparatus according to claim 5, whereinsaid velocity detection means comprises a position detector fordetecting a position of said focus lens, and a differentiation circuitfor blocking through a capacitor one of the direct-current component andultralow-frequency component in an output of said position detector.